ALD for encapsulation of scintillator crystals

Scintillators are one of the oldest types of radiation detector because initially measurements could be made with photographic film. Images could be collected, or intensity measurements made. Measurements could even be made with the human eye observing the brightness of frequency of flashes in the scintillator. Nowadays, the light output is converted into data that are processed using compact photodiodes. Scintillators come in different types, and in this edition of the blog, we look at Beneq’s ALD encapsulation solutions. Manufacturers of scintillators with applications to nuclear medicine, environmental measurements, geophysics and medium energy physics will find this information relevant to their work.

The problem with halide scintillators

Sodium (Na) and thallium (Tl) activated CsI scintillators have been widely used for ionizing radiation detection since the late 1960’s. These detectors are comparable to the widely used NaI(Tl) scintillator in terms of their energy resolution.  All these cubic alkali-halide scintillators are relatively easy to grow, making them very cost effective. Furthermore, these scintillators are rugged and less sensitive to temperature change and mechanical deformation making them ideal candidates for field applications.

Sodium Iodide doped with Thallium, NaI(Tl), has perhaps the greatest light output among the family of scintillators, convenient emission range, the possibility of large-size crystals production, and their low prices compared to other scintillation materials. NaI(TI) crystals with increased dopant concentration can be used to manufacture X-ray detectors of high spectrometric quality. These advantages make its use very attractive even when considering that Nal(TI) is highly hygroscopic and it will deteriorate due to water absorption if exposed to the atmosphere for any length of time. Consequently, NaI(TI) can be used only in hermetically sealed assemblies.

Cesium Iodide crystals doped with Thallium CsI(Tl) may not be as bright but its emission spectrum has as a peak at at 550 nm, which allows photodiodes to be used to detect the emission. The use of a scintillator-photodiode pair makes it possible to diminish significantly the size of the detecting system (due to the use of photodiode instead of PMT), and to do without high-voltage supply source. Studies show that CsI(Ti) are somewhat less sensitive than the Na activated crystals but their performance nevertheless decrease monotonically with exposure time particularly in higher humidity conditions., Once again, special attention is required for storage and packing these crystals.

Cesium Iodide doped with Sodium CsI(Na) is also widely used with performance falling somewhere in between the two previous alkali-halide crystals. High light output (85% of that of NaI(TI)), emission in the blue spectral region in coincidence with the maximum sensitivity range of the most popular photomultiplier (PMT), and substantially lower hygroscopicity in comparison with that of NaI(TI) makes this material a good alternative for NaI(TI) in many standard applications. The temperature dependence of light output has its maximum at 80°C. This makes it possible to use CsI(Na) as scintillation material at elevated temperatures. As with all previous cases, it must be encapsulated to prevent humidity induced deterioration.

The Beneq solution

Scintillator crystals can be coated with ALD technology to produce a conformal, pinhole-free barrier coating. Thin film encapsulation (TFE) is considered one of the most reliable methods to ensure protection from moisture and oxygen penetration. Metal oxide thin films such as aluminum oxide (Al2O3) and titanium oxide (TiO2) grown by atomic layer deposition (ALD) provide superior protection against moisture. Moreover, multilayered nanolaminate structures comprised of alternating layer of different materials further improve the robustness of the coating. Beneq’s  P400A and P800 batch ALD equipment is presently used to coat large scintillating plates with a high quality moisture barriers with either single of multi-layer metal oxides.

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